This invention relates to a process of treating residues from the hydrometallurgical production of zinc, particularly from the electrolytic production of zinc, in combination with an electrolytic production of zinc.
The residues which are formed in hydrometallurgical processes can be disposed of only with difficulty and still contain valuable substances. These remarks are particularly applicable to the electrolytic production of zinc. In the preparation of the zinc sulfate solution for use in the electrolytic process, the solution is neutralized and the iron is subsequently precipitated. The residue which becomes available after that neutralization and before said precipitation contains impurities, such as Ge, Ga, In, As, Cu, Pb, SiO.sub.2, Al.sub.2 O.sub.3 and also contains Zn. That residue must be removed from the process in order to prevent an enriching of said impurities in the circulating system or a contamination of the precipitated iron with said impurities because such contamination would adversely affect the utilization of said iron.
In a process known from Published German Application No. 25 40 641, the roasted concentrate is initially subjected to a neutral leaching step, the resulting ZnSO.sub.4 solution is supplied to the electrolytic process, and the leach residue is leached in two hot and acid steps. The first of the hot leaching steps is carried out under reducing conditions established by an addition of zinc concentrate. After an optional preneutralization, the leach residue left after said first hot leaching step is treated in the second step with a sulfuric acid solution which has a much higher concentration than the sulfuric acid solution used in the first step. Unreacted zinc sulfide concentrate and elementary sulfur are separated from the solution by flotation and the remaining Pb residue is also separated from the solution. The solution is subsequently recycled to the first hot leaching step. After the removal of the leach residue from the solution obtained in the first hot leaching step, said solution is neutralized with lime to a pH value of about 4.5. The resulting precipitate constitutes a residue, which consists mainly of CaSO.sub.4 and contains the impurities described previously and must be removed from circulation. The iron contained in the remaining solution is oxidized and precipitated and the solution is then recycled to the neutral leaching step. The gypsum residue which is formed by the neutralization is highly contaminated and for this reason cannot advantageously be utilized and its disposal would result in a pollution of the environment. Besides, that gypsum still contains valuable materials.